The present invention relates to method and device for controlling a multi-cylinder engine.
Conventionally, an art for suppressing an engine torque variation in idling of an engine is known. For example, JP1996-270491A discloses a speed control device of an engine which feedback-controls an idle speed with an ISC valve (idle speed control valve). The speed control device is provided with an auxiliary component load air amount control valve which controls a discharge pressure of a pump for auxiliary component load to change an intake air amount. The speed control device suppresses a variation of the idle speed due to an auxiliary component load variation, by estimating an air amount passing through the auxiliary component load air amount control valve and controlling an operation amount of the ISC valve according to a change amount of the passing air amount per unit time.
Meanwhile, with a multi-cylinder engine, when a total operation period of time of the engine becomes long, in a low engine load state (e.g., in idling), there is a possibility that the engine torque varies due to variations in performance of an ignition system and a fuel supply system of each cylinder and a valve operating system. Such a variation of the engine torque causes an increase of the engine vibration.
Thus, it can be considered to increase the idle speed so as to suppress the engine torque variation as disclosed in JP1996-270491A; however, by simply increasing the idle speed, a fuel consumption amount increases.